High-speed interface links connecting a source device to a sink device over a physical cable are typically serial communication links. Examples for such links include, but are not limited to, a high-definition multimedia interface (HDMI), a digital video interface (DVI), DisplayPort (DP), digital interface for video and audio (DiiVA), Universal Serial Bus 3 (USB3), and others. A receiver at the sink device recovers data and clock signals transmitted over the cable using a clock data recovery (CDR) circuit. Typically, the physical cable exhibits the characteristics of a low-pass filter. Therefore, the amplitude of the recovered data, received at the receiver, is attenuated and the phase is distorted. Also, the physical cable typically consists of wires which are not perfectly shielded. Thus, noise is present in the recovered data due to cross-coupling between signals from different wires.
Adaptive equalizers are used to restore signal integrity by compensating for the frequency dependent attenuation that occurs during transmission of serial data over the physical cable. However, circuitry for performing the attenuation estimation needed for adaptive equalization has been complex and difficult to implement, specifically when estimating the attenuation of data transmitted over each of two or more channels of a multi-channel serial link, in order to perform adaptive equalization of the data transmitted over each channel.
An equalizer can generally be modeled as a filter. If the cable over which a signal is transmitted has a transfer function H(s), where ‘s’ is the complex frequency, the ideal filter has the inverse transfer function H−1(s). Additionally, if noise is injected into the transmission system, the ideal filter should reject such noise as long as the noise is outside the bandwidth of the useful signal. If the noise is inside the bandwidth of the useful signal, a trade-off can be made about the degree of noise rejection together with the useful signal portion. The optimal trade-off would be such that the signal-to-noise ratio is maximized. Thus, the problem of adaptive equalization is two-fold: estimation of the inverse signal transfer function and estimation of the optimal noise rejection function.
Conventional adaptive equalizers typically examine the eye diagram of the input serial data and equalize the input data accordingly. For example, US Patent Application Publication No. 2008/0247452 to Lee, et al. (hereinafter Lee) teaches an adaptive equalizer that uses a 2-times oversampling Bang-Bang phase detector. Such a phase detector recovers the input data and generates binary timing information (Up/Down) indicating the timing of the edge clock compared to the center clock in the recovered input data. A data decode block then decodes the Up/Down timing information and the data pattern and determines whether the UP/Down timing information and the data pattern output by the phase decoder indicate a need to increase the equalization coefficients of an equalizer core. The equalizer core receives the input data and provides the equalized data, based on the equalization coefficients set for the equalizer. A bang-bang phase detector has very high jitter. It treats every positive phase shift as 180° and every negative phase shift as −180°. On the other hand, a linear phase detector (e.g., a Hogge phase detector) would measure the phase more accurately and thus provide better information for choosing the optimal equalizer.
The limitations of conventional adaptive equalizers, such as that disclosed in Lee include their limited performance and inability to efficiently equalize high rate serial data. These limitations result, in part, from the 2-times oversampling phase detector, which leads to a binary correction. Specifically, for high transmission rate, e.g., 3 Gbps and above, it is almost impossible to lock on a received signal. Thus, the phase detector in most cases would be out of range. Furthermore, the limitations of conventional adaptive equalizers prohibit the utilization of low-cost and low-quality physical cables for the multimedia interface. In addition, the length of the physical cable is limited. As with any interconnect, signal attenuation and interference of such cables increase with cable length and the signal-to-noise ratio decreases with cable length. Thus, when implementing the conventional adaptive equalizer in a receiver of a multimedia interface, low-cost, low-quality, and long distance physical cables cannot be utilized, as conventional adaptive equalizers are incapable of properly restoring receiver signals.
It would be, therefore, advantageous to provide an improved adaptive equalizer for high-speed serial links.